1. Field of the Invention
The present invention relates to an information recording system which records various information on a recording medium and particularly to a circuit for detecting defects on a recording medium during the writing operation.
2. Description of the Prior Art
In information recording system of this type, omission of recorded information is a critical problem. In order to minimize the effect of omission there have been developed various signal formatting techniques for detecting and correcting readout errors. However, the error detection and correction based on the formatting techniques has a limit of ability. An alternative method conceivable is to find (pre-check) unretrievable defects in each sector of the recording medium immediately before it is recorded, so that recording takes place without using the defective portions. This method, however, takes a long pre-checking time comparable with the recording time, and it is unfavorable for a long term mass recording. Furthermore, in some kinds of defect, e.g., a missing deposition layer on the recording medium or a partial change in the recording material, pre-checked defects are not in correspondence to defects estimated on the reproduced waveform of signals, and therefore the reliability is not satisfactory.
Still another method is to detect defects based on the recording information at a low energy level during the recording operation, but this method has a problem of a fluctuating level on the low level side. Therefore, the signal amplitude is so small that it is susceptible to the drift and the like of the circuit, resulting in an inferior detection ability.